Introduction:
AEROCE is a comprehensive multi-disciplinary and multi-institutional research program that focuses on a number of aspects of the atmospheric chemistry over the North Atlantic Ocean. A major objective of AEROCE research is to gauge the impact of anthropogenic sources on the chemical and physical properties of the atmosphere, to assess the consequences of the perturbations on natural processes including climate, and, through the use of models, to predict the longer term effects. AEROCE Phase I began in September 1987; AEROCE is now in Phase III which will extend through 1998. AEROCE research is focused in two theme areas:
The AEROCE strategy is to develop a chemical climatology of the atmosphere over the Atlantic. This is accomplished with a coordinated protocol of continuous measurements of aerosol chemical and physical properties in a network of stations: Barbados, West Indies; Bermuda; Izaña, Tenerife, Canary Islands; and Mace Head, Ireland. The Izaña site, a meteorological observatory operated by the Spanish government, is located at an elevation of 2360 m which at night is in the free troposphere. Izaña is designated as a WMO Global Atmosphere Watch (GAW) site. The AEROCE operations at Izaña are carried out in close cooperation with the Spanish meteorological service and by various other investigators associated with the ACE-2 program . The Mace Head site, located on the west coast of Ireland, about 60km WNW of Galway, is operated by University College Galway; Mace Head is also a WMO GAW site.
A subset of AEROCE protocol measurements is carried out at several other locations. Routine sampling is carried out at a coastal site at Miami, Florida. A station on Heimaey, an island off the south coast of Iceland, is operated in cooperation with the Iceland meteorological service. As a part of the ACE-2 program, a station was established at the top of a 40m light house at Punta del Hidalgo on the north coast of Tenerife. Recently, AEROCE activities were initiated on Sal Island (Cape Verde Islands) in cooperation with a group at the University of Paris (L. Gomes and G. Bergametti).
The integrated activities across this network have provided an unprecedented picture of the temporal and spatial variability of a number of important gas, aerosol, and precipitation constituents across the entire North Atlantic. This report focuses solely on the Aerosols and Climate theme.
AEROCE Strategy for Assessing Aerosol Impacts on Climate
The AEROCE program follows two sampling strategies:
(1) a continuous sampling program that develops a climatological data set on the chemical, physical, and radiative characteristics of aerosols;
(2) intensive field programs that incorporate a much more extensive measurement protocol with a strong emphasis on aerosol size distribution measurements and radiative properties.
Long-term observations allow us to statistically characterize these properties and their temporal and spatial variability. Measurements taken during intensive field campaigns focus on specific processes in the context of case studies. In both modes, long-term and intensive, we test the completeness of our knowledge of aerosol properties by comparing the measured radiative parameters with those computed with aerosol radiative models. A major strength of AEROCE is that the sites are exposed to a wide variety of atmospheric conditions with winds that can carry aerosols from sources in North America, Europe, and Africa; consequently, aerosol characteristics vary over wide ranges. These factors enable us to more readily associate changes in radiative properties to changes in chemical and physical properties of aerosols. Meteorological studies carried out in AEROCE enable us to associate these changes with specific source regions and source types.
Continuous Protocol
Aerosol Chemical Measurements: We collect, daily, high volume, bulk aerosol samples under selected meteorological conditions (to minimize the possible impact of local sources). These samples are analyzed for various soluble species derived from natural and anthropogenic sources, e.g., nss-SO4=, NO3--, NH4+, sea-water species. Various trace elements serve as indicators of source type and/or region. Aluminum is used to estimate mineral aerosol concentration; other elements (e.g., Sb, Zn, Rb, Se) can serve as tracers for various anthropogenic sources and biomass burning. Finally, two radionuclides, 7Be and 210Pb, are used as tracers for upper troposphere/stratosphere air and continental boundary layer air, respectively.
Aerosol Physical Properties: We make continuous measurements of: (1) aerosol light scatter using integrating nephelometers. The air stream is heated (to reduce the RH to under 50%) and drawn through a 10 um impactor; an automatically switchable 1 um impactor is periodically inserted into the line so as obtain data that is specific to the large-particle and sub-micron size ranges; (2) aerosol composition in two size ranges: 1-10 um diameter and the fraction below 1 um (i.e., the same size fractions viewed by the nephelometer); (3) total aerosol number concentration; and (4) total aerosol light absorption coefficient. The concentration of organic components and black carbon is estimated using a step-wise oxidation procedure and using non-dispersive IR measurement of the evolved CO2.
Radiation measurements: The sites at Barbados, Bermuda, Miami, Tenerife, and Sal Island currently make continuous radiation measurements with two types of instruments: rotating shadowband radiometers (RSR) and automatic solar-sky scanning radiometers (ASSR). The RSR measures radiation in 5 passbands and an open channel (for total downwelling irradiance), yielding diffuse, direct and total solar irradiance. This instrument is used extensively in the DOE/ARM program. The RSR's at the AEROCE sites are deployed by investigators from several different institutions. The ASSR (CIMEL) instruments automatically make direct sun measurements every 15 minutes at 340, 440, 670, 870, 940 and 1020 nm; sky radiance and polarization measurements are made hourly. The ASSR's are deployed by various groups but they are all integrated into the Aerosol Robotic Network (AERONET) under B. Holben (NASA, Goddard) who maintains an on-line central data base for all instruments. Routine products are spectral aerosol optical depth, size distribution, phase function, asymmetry factor and precipitable water.
Other Measurements: A variety of other measurements are made at AEROCE sites. These include continuous measurements of O3 and CO. These data assist greatly in the interpretation of the aerosol data. In addition, meteorological data are obtained and meteorological support is provided (in the form of isentropic trajectories and other meteorological products) as a routine component of the AEROCE protocol.
Intensive Protocol
The aerosol intensives employ more sensitive aerosol aerosol/radiation instrumentation than that used in the continuous program. Emphasis is placed on measuring aerosol size distributions using state-of-the art instrumentation. Because many of these instruments have averaging times on the order of minutes, we obtain much higher temporal resolution which allows us to interpret changes in aerosol properties in terms of rapidly changing meteorological conditions. Consequently, specific aerosol properties can be more definitively associated with specific transport conditions.
Aerosol Physical Properties: We continue to make the measurements listed in the Continuous Program (as described above). In addition we measure: (1) aerosol total integrated light scattering and back scattering using a high-sensitivity integrating nephelometer (three spectral bands) that incorporates a back-scatter shutter (TSI Model 3563); and (2) aerosol size distributions with several instruments that cover the size range from 3nm to 15 um diameter.
Aerosol Chemical Measurements: In addition to the measurements that we make under the continuous protocol, we measure: (1) size distributions using a variety of impactors; (2) continuous total aerosol mass using tapered element oscillating microbalances; (3) annular diffusion denuders and filter packs.
During the past several years intensive field campaigns have been carried out at Bermuda, Barbados and Tenerife. In preparation for the ACE-2 campaign, we have participated in intensive field studies in cooperation with scientists from the CEC Joint Research Center in Ispra, Italy, during the summers of 1994 through 1996.
Some Examples of AEROCE Results
The measurements made over the past few years clearly show that the atmosphere over the North Atlantic is highly impacted by materials transported from continental sources. Table 1 presents the mean concentrations of some major aerosol species, based on at least several years of data. The concentrations of species such as non-sea-salt sulfate (nss-SO4=) and nitrate (NO3--) are substantially higher than concentrations in remote ocean regions (e.g., the southern oceans).
Table 1. Concentrations of major aerosol constituents measured at AEROCE sites in the North Atlantic based on multi-year continuous sampling.Station Lat. Long. Al Dust+ NO3-- nss-SO4= NH4+
deg.N deg.W ng/m3 ug/m3 ug/m3 ug/m3 ug/m3
Mace Head 53.32 9.85 38 0.47 1.49 2.03 0.91
Bermuda 32.27 64.87 447 5.6 1.06 2.19 0.31
Izaña 28.30 16.50 1783 22.3 0.77 0.92 0.33
Miami NA 5.6* 1.87 2.34 0.61
Barbados 13.17 59.43 1164 14.6 0.53 0.78 0.11
+ Dust concentration computed from Al based on a crustal abundance of 8% except for Miami data which is based on weights of filter samples ashed at 500deg.C after extracting with water.
* Ash weight, 96 months of data.
The effect of transport from the North America is clearly evident at Bermuda (Figure 1) as reflected in the large day-to-day variability in the concentrations of NO3-- and nss-SO4=. A seasonal pattern is also evident, with maximum concentrations in the winter and spring, when the region is often dominated by westerly flow. In contrast, during the summer, southerly winds are prevalent and the concentrations of NO3-- and nss-SO4= are subtantially lower.
Figure 1. Aerosol concentrations on Bermuda, 1989-1993. Data are based on daily samples collected during on-shorewinds. AEROCE maintains two sampling sites on Bermuda, one on the west end at Tudor Hill and one on the east endat David's Head. Thus, we can collect samples under all wind conditions. The data in the figure are composited from the samples collected at both sites. (Data courtesy of D. Savoie, Univ. of Miami).
One of the most striking features of the aerosol records at many AEROCE sites is the frequent presence of high concentrations of North African dust. As can be seen in Table 1, the concentrations of dust are quite high at Izaña and Barbados, but they are also substantial at Bermuda and Miami. There is a very clear seasonal cycle in dust transport (Figure 2), with maximum concentrations occurring in the summer at all sites. Note that the dust concentrations at Barbados are comparable to those measured at Izaña; although peak concentrations tend to be higher at Tenerife, the dust transport to Barbados occurs over a longer period during the year. Because of the more northerly location of Tenerife relative to Barbados, dust transport does not become persistent until mid-summer, generally in July. This feature has implications regarding the activities in ACE-2 which begins in mid-June and extends through July. According to the dust climatology of Tenerife, the ACE-2 region should be relatively dust-free in the early weeks and then dust levels should increase sharply.
Figure 2. Daily aluminum concentrations in aerosols collected at Barbados,West Indies, Bermuda and Izana Observatory. Samples are only collectedat night-time during downslope wind conditions. The mineral dust concentration can be estimated by multiplying the Al values by 12.5, assuming an average Al concentration of 8% in soils. (Data courtesy of R. Arimoto, Univ. of Rhode Island).
The large variability of aerosol concentrations and properties at Izaña is illustrated in Figure 3 which shows data from July 1995. The air was extremely clean until mid July when there was a sudden influx of African dust. Note that the concentrations of all species increased sharply with the dust. Previous studies have shown that the NO3--, nss-SO4=, and NH4+ associated with the dust was most likely derived from sources in Europe.
Figure 3. Daily aerosol concentrations at Izana Observatory, July 1995.Samples are normally collected only at night-time during downslopewind conditions. (These are plotted between the dates in the figure.)During July, some day-time upslope sampling was doneas well; these are also shown in the figure. The upslope windconcentrations do not appear to be substantially different from the night-time samples, especially during dusty periods. (Data courtesyof D. Savoie, Univ. of Miami).
Summary
The AEROCE studies show that aerosols over the North Atlantic are greatly affected by transport from continental sources. The temporal and spatial variability of aerosols over this region is quite complex because of the abundance of sources, natural and anthropogenic, that border the North Atlantic and because of the complex meteorology. Although we have developed some appreciation of the factors affecting aerosol concentrations over the Atlantic, we are far from a quantitative understanding of these processes. Our knowledge about the vertical distributions of aerosols is especially limited. Focused intensive field programs such as TARFOX and ACE-2 coupled with longer-term programs such as AEROCE will do much to improve our knowledge about these processes and the role that aerosols play in climate processes.
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